Just a day old, their bellies were full of milk, im not sure how they passed :/ they even have a heating pad under the nesthow recent? Did the babies have milk bellies or bands up to 3-4 days old? She may not have been producing milk. I'm sorry for your many wee losses
My room gets very cold but i'll turn it off for a while, the mom is taking very good care of the remaining one so it should be okay.Unless your place is very cold I wouldn't use a heating pad. Heat kills rats faster than cold and wee babies wouldn't be able to move if it became too warm.